1. Field of the Invention
The present invention relates to a device and method for diagnosing a fuel pressure sensor for detecting common rail fuel pressure.
2. Description of the Related Art
A control computer of pressure-accumulating type fuel injection control devices comprising a so-called common rail uses a regulating valve to execute a feedback control on the discharge quantity of an injection pump (supply pump) based on fuel pressure sensor (common rail pressure sensor) information obtained while the engine is running in such a way as to ensure a target common rail pressure value established in accordance with the engine revolutions and load and so on is produced. To that end, a common rail pressure sensor serves a very important function.
If a characteristic shift (drift) in the output voltage of this common rail pressure sensor occurs, the feedback control executed on the common rail pressure while the engine is running will be based on an erroneously perceived value and, in turn, the actual pressure will be higher or lower than the virtual pressure (pressure detected by the common rail pressure sensor).
As a result, when characteristic shift of a common rail pressure sensor occurs, irrespective of the injector and so on being instructed by the control computer to generate an identical target injection quantity, the actual injection quantity changes due to the abnormal common rail pressure and, as the desired fuel injection quantity cannot be produced, leads to exhaust emission and performance deterioration.
A method based on detecting short-circuiting or disconnection of a sensor circuit is conventionally used as a method for monitoring and diagnosing the normality/abnormality of a common rail pressure sensor.
In another method for indirectly diagnosing a common rail pressure sensor, because the common rail pressure limit value in terms of feedback control is reached and, in the end, compliance therewith is impossible when a marked characteristic shift (drift) in the output value of a common rail pressure sensor occurs while an engine is running, the diagnosis utilizes a virtual increase or reduction in the real common rail pressure as detected by the common rail pressure sensor with respect to a target common rail pressure value.
In addition, Japanese Unexamined Patent Application No. 2001-173507 proposes a means for detecting the occurrence of offset of a fixed level or greater of a common rail pressure sensor characteristic value. In Japanese Unexamined Patent Application No. 2001-173507, common rail pressure sensor characteristic detection is performed when the common rail pressure drops to atmospheric pressure such as when the vehicle is stationary. More specifically, in Japanese Unexamined Patent Application No. 2001-173507, engine stoppage for a fixed time is evaluated on the basis of a drop in water temperature, and the common rail pressure sensor is diagnosed as abnormal when, in a state in which sufficiently rest and stabilization of the common rail pressure has occurred, the ignition key is ON, and the engine rotation is stopped prior to it being started, the common rail pressure, which should be essentially equivalent to the atmospheric pressure, is high.
However, the following problems are inherent to the diagnosing method described above.
First, in the conventional method for indirectly diagnosing a common rail pressure sensor, because the diagnosis is performed while the supply pump is being actuated, aside from simple disconnection/short-circuiting failure, both failure of the common rail pressure sensor and failure of the supply pump must be considered as causes of a lack of equivalence between a target common rail pressure and common rail sensor detected pressure and, as a result, it is difficult to specify which component part is the cause thereof and vehicle inspection takes a significant amount of time.
Furthermore, in Japanese Unexamined Patent Application No. 2001-173507, because the diagnosis is performed when the ignition key is ON subsequent to a sufficient stabilization having elapsed, the engine rotation must be stopped for at least the time required for the diagnosis even though the ignition key is ON. For this reason, there is a concern that diagnosis will be precluded from being performed when a driver switches ON the ignition key and immediately thereafter actuates the start causing the engine to rotate.
Furthermore, in Japanese Unexamined Patent Application No. 2001-173507, because the engine cannot be started when a shift in the common rail pressure sensor characteristic occurs and the shifted characteristic is voltage offset to the high side and fixed, this presents an opportunity for a diagnosis to be performed. On the other hand, when the shifted characteristics are offset to the low side, there is a possibility that the engine will be started and run in an abnormal state and, in turn, a concern that a diagnosis will not be able to be performed.